An electro hydrostatic actuator (“EHA”) is a self-contained actuator run and controlled by its own electrically powered motor-pump assembly, thereby eliminating the need for a separate hydraulic pump. In aerospace applications, EHAs are typically a fly-by-wire (“FBW”) device that is operated by control electronics. The hydraulic actuator of an EHA may be used to move aerodynamic surfaces such as a flap on an aircraft wing. EHAs may replace conventional hydraulic actuator systems in an aircraft for various operations, including e.g. landing gear retraction/extension, steering, braking, and fluid control.
Especially when EHAs are used to control components of aircraft operation, it is important to provide redundancy in the event of electrical power generation failure or control path electronics failure. It has previously been proposed, for example as seen in US 2013/0067898, for an EHA to comprise a failsafe hydraulic circuit that includes additional hydraulic oil paths allowing communication between the jack chambers of the hydraulic actuator and a solenoid “mode” valve that can be switched to connect the additional hydraulic oil paths and allow for a bypass flow of hydraulic oil outside the electrically-powered hydraulic pump, e.g. in an emergency state of power loss. In such an emergency state, the hydraulic actuator is still able to provide a damping force to its aerodynamic surface. This damping effect is switched on or off by the additional solenoid valve in the hydraulic circuit.
Previous developments in this field have included systems configured to open a bypass flow path through the hydraulic pump when the pump is not driven by the electric motor in a damping mode of operation. By opening a bypass flow path within the pump, the damping mode of operation is integrated into the pump rather than requiring an additional solenoid “mode” valve in the hydraulic circuit between the pump and the actuator.
The present disclosure seeks to provide improvements in arrangements that involve incorporating a bypass flow path through the pump of an electro-hydrostatic actuator.